1. Field of the Invention
The present invention relates to a sensor for determining position or displacement of a movable mechanical part, especially for detecting reciprocating motion of a mechanical part, such as needle valve, and, more particularly to a microwave sensor for determining position or displacement of a movable mechanical part including a cavity resonator for input microwaves of a predetermined frequency, which comprises at least partly metal walls bounding a cavity into which the mechanical part extends, and an antenna for receiving the microwaves fed into the cavity.
2. Prior Art
Sensors of this type are used particularly in measuring relatively short displacements or strokes of deflectable bodies, such as in detecting the valve needle stroke in injection valves for motor vehicles. In order to detect the slightest deflection motions within the tightest possible space known optical or magnetic sensors are constructed, which as a rule are structurally very complicated and entail high cost.
From European Patent Disclosure EP 0 427 882 B1, for instance, a sensor of the above-defined type is known in which a permanent magnet, as a deflectable body, moves in the range of a magnetic-field-sensitive sensor.
Particularly for measuring needle strokes in an injection nozzle system for motor vehicles with improved emissions values, the stroke sensor should be disposed directly in the nozzle holder, where the installation space is extremely tight.
It should be possible to detect the needle stroke even at an initial stroke of approximately 20 xcexcm; the ambient conditions include a relatively wide temperature range from approximately xe2x88x9240xc2x0 C. to 140xc2x0 C. and the resistance to jarring should be assured up to 80 g.
It is an object of the present invention to provide an improved position or displacement sensor, especially for accurately determining the position or displacement of a reciprocating mechanical part, such as a valve needle, in a very tight space, which does not have the above-described disadvantages of known optical or magnetic sensors.
This object and others, which will be made more apparent hereinafter, are attained in a microwave sensor for determining the position or displacement of a reciprocating mechanical part, which includes a cavity resonator for microwaves of a predetermined frequency. The cavity resonator comprises at least partly metal walls bounding a cavity dimensioned for the microwaves and into which the reciprocating mechanical part extends, and an antenna for receiving the microwaves input to the cavity.
According to the invention the mechanical part has a metal end, the cavity of the cavity resonator is dimensioned for resonance of an H110 mode of the microwaves in the cavity, the cavity is circular-cylindrical, the end of the mechanical part extends into the cavity, the cavity has a diameter between 2.405xe2x80xa2c/xcfx80/f and 1.841xe2x80xa2c/xcfx80/f, wherein f represents the predetermined frequency of the microwaves fed into the cavity and c represents the speed of light, and the cavity has a height of up to 0.4xe2x80xa22.405xe2x80xa2c/xcfx80f, so that it is even possible, for example, to detect a displacement of about 20 microns.
A position or path sensor according to the invention is advantageous because in a simple way, a cavity with small dimensions can be provided, and the mechanical part whose position or path is to be detected protrudes into this cavity, the cavity being designed as a microwave resonator. The sensor of the invention makes use of the propagation conditions, known per se, of electromagnetic waves in the microwave range (approximately 38 GHz) in metal hollow conductors. By reflection and superposition of simple shallow waves, hollow conductor waves are created that can also be understood as interference waves. Because of the geometric conditions in a rectangular hollow conductor, a so-called H10 hollow conductor wave forms as the fundamental hollow conductor wave in question, which in the course of propagation of the hollow conductor wave has a defined field line pattern with respect to the electric and magnetic field lines. In a cylindrical hollow conductor, an H11 hollow conductor wave correspondingly forms as the fundamental wave; this is the only mode capable of propagation, if the cavity is selected to be sufficiently large.
In the stroke sensor of the invention, a resonator for the hollow conductor waves is formed of the kind described by H.-G. Unger, xe2x80x9cElektromagnetische Theorie fxc3xcr die Hochfrequenztechnikxe2x80x9d [Electromagnetic Theory for High-Frequency Technology], Part I, Second Edition, Hxc3xctig Verlag, pp. 292-294. Such a resonator is created if in a circular-cylindrical hollow conductor, for instance, both ends are short-circuited with conductive plates. The resultant natural vibrations of the resonator fit precisely into the resonator with an integral multiple of half their propagation wavelength. If a metal needle now protrudes into this cavity resonator, the result is a different field distribution in the cavity resonator, and thus a measurable change in the resonant conditions.
According to the invention, the cavity into which the needle to be detected protrudes is advantageously designed geometrically such that the thus-formed resonator is operated at a specified operating frequency (such as 38 GHz) with a natural vibration in an H110 mode. An oscillator for generating the operating frequency, which can also be integrated in combination with a so-called double ratrace ring to replace a conventional insulator and a circulator in the housing of the needle to be detected, is known in a similar form, for instance from EP 0 685 930 A1. The so-called double ratrace ring is an arrangement of two couplers with a mixed diode in a ratrace coupler form.
In a preferred exemplary embodiment, the transmitting output of the oscillator is advantageously coupled into the needle housing, embodied as a cavity resonator, via a wirelike probe acting as a transmitting and receiving antenna or via a rod antenna. Some of the transmission power is supplied, via one arm of the ratrace ring, to a mixer, and the reception signal decoupled from the oscillator is delivered to the second arm of the ratrace ring. Upon mistuning of the cavity resonator by a needle motion, the change in a zero crossover phase can be picked up as a low-frequency signal at one output of the ratrace ring.
The above-described circuit can be constructed in a simple way as a hybrid circuit using so-called microline technology (MIC or monolithic integrated circuit) with an integrated GaAs-MMIC circuit (gallium arsenide microwave monolithic integrated circuit). The dimensioning rules for designing the cavity resonator, using the calculation methods known for instance for cylindrical hollow conductors, result in a maximum diameter dmax=2.405xe2x80xa2c/xcfx80/f and a minimum diameter dmin=1.841xe2x80xa2c/xcfx80/f, where f is the operating frequency and c is the speed of light. The height of the cylinder should be at maximum 0.4xe2x80xa2dmax. In a different geometrical design of the cavity resonator, for instance with regular polygons such as a hexagon or octagon and so forth, the dimensioning rules should be adapted accordingly.
In a preferred exemplary embodiment, for an operating frequency of 38 GHz, a resonator diameter of 5.5 mm and a cylinder height of 2 mm result; the reciprocating motions of the needle amount approximately to a range of from 0.2 mm to 1.5 mm into the resonant chamber. The cavity resonator can be disposed symmetrically or asymmetrically with respect to the end of the needle to be detected. Any required fuel (Diesel) recirculation is also possible through the cavity, if the requisite bore for the purpose, in the embodiment described (where f=38 GHz and xcex5R=1) is selected to be no greater than 2.5 mm, and the cavity size is corrected to suit the dielectric constants.
As an alternative to the above-described inputting of microwave energy into the resonant chamber, the input can also be effected radially or orthogonally via a coupling loop coaxial cable or a hollow conductor. It is also possible to include a ceramic, glasslike or plasticlike carrier material for the antenna (such as Al2O3, BaTiO3, quartz glass, or plastics such as PE, PC, PP, PTFE, etc.).